Rayden

Thank you very much for your detailed reply. You are completely correct, that is what I did, and I was indeed fearful that I would have to reinstall WIndows. I will resist using register editor. Having said that though, there did seem to be an enormous amount of junk there presumably from apps etc that I've deleted etc. However, i suppose that its not doing any harm, apart from taking up a bit of space. Once again thank you.

I have just installed CCleaner and run it to clean up my system. That all seemed to work. Now, when I try to run Command Prompt as administrator, nothing happens. Also the start button now does not work. I fortunately made a restore point before making the changes, and I think I've now managed to get the system working again. However, using CCleaner now seems rathre dangerous or maybe I've misused it. Can you please advise how I avoid such disasters?